No cosmological model is complete without a credible theory for
the formation of the structure that is observed in the Universe.
This structure, fully described in Chapter 3,
ranges from very small-scale (e.g., globular clusters and
dwarf galaxies), through bigger scales (e.g., galaxies and clusters
of galaxies) to the largest scale (e.g., large voids, superclusters,
clusters of superclusters, great walls). While a wide variety of
structure formation models can and have been considered, the
basic idea which has persisted since the time of Newton is
gravitational instability which amplifies the growth of density
fluctuations. While other scenarios are possible and will be
considered here briefly, gravitationally
instability has the great virtue of being 1) the only known
long range force/process than can aggregate matter and 2) is
physically well understood. Gravitational instability is therefore
the dominant paradigm for understanding structure formation and we
devote the first part of this chapter to discussing it in some detail.
After this discussion we will focus on statistical methods of characterizing
the distribution of structure, possible scenarios for structure formation,
and finally on the available observational constraints on competing
cosmogenic scenarios.